Answer 1

My guess is that they figured Swift is good enough considering it's a more modern language to develop for mac/iOS compare to Objective-C. However, there are reasons why I tend to disagree with this position:

1. Apple isn't gonna drop Obj-C any time soon
2. Unless I'm mistaken, Swift is actually still missing a few features compared to Obj-C.
3. The course here on Sololearn covers Swift 4, but Swift 5 was released 9 months ago.
4. Swift 4 is not ABI stable but Swift 5 is. This means code written in Swift 5+ will not break from future updates. Code written in Swift 4 (or pior versions) can (and most likely will) break.

Back to your question though, I guess they figured Obj-C will be replaced by Swift. As outlined above, you're correct and they should add an Obj-C course (and upgrade the Swift course to version 5).

Disclaimer: I haven't looked at the Swift course myself.